It should be understood that any discussion of the background art throughout the specification should in no way be considered as an admission that such background art is prior art, nor that such background art is widely known or forms part of the common general knowledge in the field of the invention.
Due to increasing social demands to protect the environment and eliminate or reduce environmental pollutants there has been an increased interest in the use of photocatalysts to decompose pollutants. Photocatalysts are able to decompose pollutants, in particular organic pollutants in water and air, but usually only when irradiated with light at a suitable wavelength. Photocatalysts are also used to produce hydrogen and oxygen from water, via a splitting reaction.
Photocatalysts generally require irradiation with light in the UV spectrum (having a wavelength of between 250 to 400 nm) in order to provide any catalytic effect. UV-light comprises approximately 5% of the solar spectra. Accordingly, the use of photocatalysts requires the use of an external source of UV light radiation. Thus, it is believed that currently known photocatalysts are not typically energy efficient. In addition, in order to achieve increased photocatalytic performance, photocatalysts usually require very small particle size and as a result may be difficult to recover.
Production of photocatalysts which are catalytically effective in the visible light spectrum (having a wavelength of between 400 to 700 nm) have been described. Photocatalysts which show some activity in the visible light spectrum include titania photocatalysts which are described in Asahi, R., et al, Science, 2001, 293, 269. However, only a very small shoulder absorption in the visible light range in this type of material was demonstrated.
In Chen X. et al, Chem. Rev., 2007, 107, 2891-2959, there is also described titanium dioxide nanomaterials which are considered ineffective as a photocatalyst.
Accordingly, the applicant believes that these photocatalysts suffer from the disadvantage that they are ineffective, expensive to produce and/or difficult to recover.
The invention seeks to overcome these disadvantages or to provide an alternative to the prior art.